Absorbent structures comprising a capillary absorbent substrate and having an osmotic absorbent applied thereto are known in the art. As used herein, a "capillary" absorbent absorbs liquids, such as water, by capillary attraction of the liquids due to the thermodynamic force of attraction between a liquid and the solid surface of the capillary medium. In contrast, as used herein, an "osmotic" absorbent absorbs liquids deposited thereon by transfer of the liquids across the periphery of the osmotic absorbent, forming a gelatinous substance which imbibes the liquids. As used herein, an "absorbent structure" refers to materials which, in combination, absorb liquids by both osmotic and capillary absorptions.
The osmotic absorbent may be made from monomers selected from the group consisting of acrylic acid, starch grafted acrylate co-polymers, etc. Such osmotic absorbent materials are commonly used as absorbent gelling materials or superabsorbers in disposable absorbent articles such as diapers and sanitary napkins. The osmotic absorbent may be applied to the capillary substrate in the form of a liquid precursor, to be later cured into an osmotic absorbent.
The capillary absorbent may be provided in the form of a substrate, for the osmotic absorbent to be later applied thereupon. Typically the capillary substrate is a generally planar, almost two-dimensional material, such as paper, nonwoven fabric, woven fabric, or even formed film.
Generally, the osmotic absorbent material may be applied to the capillary substrate as a fluid precursor, such as a liquid monomer, then crosslinked to form an absorbent polymeric material. Usually, the liquid precursor is applied to the capillary substrate in a fluid form and comprises some form of acrylic acid and acrylate salts.
Typically, the liquid precursor is applied to the capillary substrate by spraying, impregnation, etc. to provide a uniform coating thereon. Other teachings in the art suggest discontinuous applications of the liquid precursor to the capillary substrate through brushing, roller coating, etc. Once the liquid precursor is applied to the capillary substrate, the liquid precursor may be crosslinked through elevated temperature., irradiation, etc., to form the osmotic absorbent.
Examples of such attempts in the art include U.S. Pat. Nos: 4,008,353 issued Feb. 15, 1977 to Gross et al.; 4,061,846 issued Dec. 6, 1977 to Gross et al.; 4,071,650 issued Jan. 31, 1978 to Gross; 4,835,020 issued May 30, 1989 to Itoh et al.; 4,842,927 issued Jun. 27, 1989 to Itoh et al.; 4,865,886 issued Sep. 12, 1989 to Itoh et al; 4,892,754 issued Jan. 9, 1990 to Itoh et al.; 5,079,034 issued Nov. 21, 1988 to Miyake et al. and Great Britain Patent 1,452,325 published October, 1976 in the name of Triopolis.
However, difficulties can arise in the prior art method of applying the liquid precursor to the capillary substrate. For example, it is difficult to spray the liquid precursor onto the capillary substrate in a precise pattern. Printing the osmotic absorbent onto the capillary substrate may result in a pattern having greater definition and precision than obtainable by spraying, but requires a printing roll having gravure plates or raised protuberances. Printing rolls having gravure plates or raised protuberances limit the pattern of the applied osmotic absorbent to that pattern corresponding to the gravure plates or the protuberances of the printing roll, regardless of what pattern may be desirable for a particular substrate.
This problem may be overcome by providing a plethora of printing rolls, one for each desired pattern. However, once the drying belt is fixed, such provision increases the expense of the apparatus to a point where it may not be economically feasible to provide a gravure roll or flexographic roll for each desired pattern if only a short production run is desired.
Accordingly, it is an object of this invention to overcome the problems presented by the prior art. Particularly, it is an object of this invention to provide an osmotic and capillary absorbent structure which resists gel blocking and which more efficiently utilizes the entire capacity of the absorbent structure. Further, it is an object of this invention to provide an absorbent structure which is more easily and precisely manufactured than those according to the prior art. Finally, it is an object of this invention to provide a process for manufacturing an absorbent structure having the osmotic absorbent registered with discrete regions of the capillary substrate.